Into the depths of the sea!

(Thomas Boston, "Human Nature in its Fourfold State")

The sinner outside of Christ is bound over to the wrath of
God; he is under an obligation in law to go to the prison of
hell, and there to lie until he has paid the utmost farthing.

"There is therefore now no condemnation to those who
 are in Christ Jesus." Romans 8:1

The believer's sins are pardoned, the guilt of them is removed.
The bond obliging him to pay his debt is canceled. God the
Father takes the pen, dips it in the blood of His Son, crosses
off the sinner's accounts, and blots them out of His debt-book.

Being united to Christ, God says, "Deliver him from going down
to the pit; I have found a ransom!" Job 33:24. The sentence of
condemnation is reversed, the believer is absolved, and set
beyond the reach of the condemning law. His sins, which before
were set before the Lord, Psalm 90:8, so that they could not be
hidden—God now takes and casts them all behind His back, Isaiah
38:17. Yes, "You will tread our sins underfoot and hurl all our
iniquities into the depths of the sea
." Micah 7:19.

What falls into a brook may be retrieved—but what is cast into
the sea cannot be recovered. But there are some shallow places
in the sea; true—but their sins are not cast in there—but into the
depths
of the sea. The depths of the sea are devouring depths,
from whence their sins shall never come forth again. But what
if they do not sink? He will hurl them in with force, so that they
shall go to the bottom, and sink as lead in the mighty waters
of the Redeemer's blood!

They are not only forgiven—but forgotten, Jer. 31:34, "I will
forgive their iniquity, and I will remember their sin no more."
And though their after-sins do in themselves deserve eternal
wrath, and do actually make them liable to temporal strokes,
and fatherly chastisements, according to the tenor of the
covenant of grace, Psalm 89:30-33—yet they can never be
actually liable to eternal wrath.